With the introduction of the silicon planar technology, not only the active components like NMOS transistors (N-type metal-oxide-semiconductor transistors), PMOS transistors (P-type metal-oxide-semiconductor transistors) and bipolar transistors, but also the passive components like resistors are required to qualify with the silicon planar technology. The use of polycrystalline silicon, or polysilicon as it is commonly known, as a resistor, is well known in the fabrication of semiconductor devices. Generally polysilicon resistivity either increases or decreases with increasing temperature. This rate of increase is referred to as a temperature coefficient of resistance. On one hand, a precise resistor requires not only the property of stability and low-scattering, but also a relative high sheet resistance to keep the resistor as small as possible. For high-ohm resistors, which can be simply achieved by series connection of square sheets with sheet resistance of 1 kOhm/square, and the typical minimum critical dimension of such square sheet is in a range of 0.5 μm-1 μm (for example, a 500 kOhm resistor would be built up from 500 pieces of such square sheets), the resistance value of such high-ohm poly resistors produced in a conventional way varies too much with regard to the temperature, i.e. the temperature coefficient of the high-ohm poly resistors is too big, which results instability of the poly resistors. For these reasons, there is a need for minimizing the temperature dependence of a polysilicon resistor.
On the other hand, polysilicon resistors are utilized for a variety of applications, e.g. used as temperature sensors in semiconductor devices. Power transistors such as DMOS transistors (double diffused metal oxide semiconductor transistors) find multiple applications in semiconductor applications. During operation of the power transistors, a wide variety of switching states occur, in which in part very large power losses are converted into heat. Such switching states associated with large power losses are critical since the temperature rises greatly in this case and the power transistors can be destroyed by overheating. In order to protect the transistors against damage in such critical switching states, temperature sensors are often used. Ideally, the temperature sensors are positioned as close as possible to or in the cell array of the power transistor in order that a temperature rise on account of energy loss converted into heat is detected early and rapidly and that the power transistor is turned off in good time before self-destruction on account of overheating by an auxiliary circuit such as a logic circuit. In this case, a resistor situated in the cell array of the power transistor can be used as a rapidly reacting temperature sensor. The temperature sensor changes its absolute resistance value with temperature in the characteristic manner, in which case it is possible to derive a turn-off signal for turning off the power transistor when a defined maximum permissible resistance value is reached. Therefore, an obvious variation of resistance according to temperature increasing is required for such polysilicon resistance temperature sensors. For these reasons, there is a need for enlarging the temperature dependence of a polysilicon resistance temperature sensor in certain applications. However, this concept with a resistance temperature sensor often fails in practice because of excessively large manufacturing variations with which a resistance temperature sensor of this type can be produced, since the absolute value of the resistance cannot be used as a turn-off threshold meaningfully. For these reasons, there is a need for minimizing the manufacturing-variations dependence of such polysilicon resistance temperature sensor.
Thus, there is a need in the art to provide methods for manufacturing poly resistors precisely and in the meantime, for manufacturing poly resistors with well-controlled temperature coefficients.